I have downloaded and installed tomcat 9 to my machine, but on Eclipse > Window > Preferences > Server > Runtime Environment, after clicking on Add, I see only Tomcat 7 and Tomc
There are two things to note here:
Tomcat 9 is not released yet. Not a final version that is. If you check the official website, you will see that the only available version is an M1 version. Hence there is no support from Eclipse WTP for this early version. It will come soon, but not yet.
Tomcat 9 is not the only version that explicitly supports Java 8. If you check this page, you will see that "Supports Java version: 8 and later", which in this context means, that it officially support only Java 8 and above. It does not mean however this is the only one supporting Java 8. Theoretically you should be able to use any Tomcat version, in practice a Tomcat 8 will work perfectly well, since it support Java 7 and above. The majority of people with a Java8 + Tomcat combination are using Tomcat 8. Tomcat 8 also has Eclipse WTP support.